Global Mid-ocean Ridge Basalts Research Articles

Prediction of CO2 content in mid-ocean ridge basalts via a machine learning approach

Abstract One of the primary locations of mafic magma production on Earth is the global mid-ocean ridge system. The basalts erupted along ridges probe the upper mantle and can be used to explore the deep carbon cycle. However, mid-ocean ridge basalts (MORBs) degas heavily during magma ascent. Some incompatible-trace-element–depleted and –enriched MORBs avoid heavy degassing, and show a narrow range of CO2/Ba, which have been used to reconstruct the pre-eruptive CO2 content of primitive MORB. With an increasing amount of data, however, it has become apparent that the CO2/Ba ratios of MORBs vary significantly. We compiled a data set of the geochemical compositions of MORB glasses and melt inclusions that are not degassed significantly and used a supervised machine learning model to accurately predict CO2 contents of individual samples from the concentrations of selected elements. This approach reveals that predicted CO2 contents and CO2/Ba ratios of global MORBs are highly variable, highlighting the significance of mantle heterogeneity, which can be attributed to the interactions with deep-sourced plumes or recycled crust (oceanic crust with or without sediments). Our findings underscore the potential of machine learning as a powerful tool for investigating the intricate interplay between carbon, mantle composition, and Earth's long-term geological processes.

Concordance of V-in-olivine and Fe-XANES oxybarometry methods in mid-ocean ridge basalts

Oxygen fugacity (fO2) is a fundamental parameter controlling the behavior of redox-sensitive elements, as well as the speciation of degassing volatiles in igneous systems. Therefore, the average fO2 of typical mid-ocean ridge basalts (MORB) is important to constrain for models of atmospheric and mantle chemical evolution. Although there are several independent oxybarometry methods, in recent years, the Fe X-ray absorption near edge spectroscopy (Fe-XANES) oxybarometry of volcanic glasses has become the gold standard for evaluating the fO2 of modern lavas. Other oxybarometry methods, such as the V-in-olivine method, are useful for samples in which fresh glass is not preserved, but it is not clear whether the two methods give comparable results when applied to the same samples due to the lack of inter-calibration studies. Presented here are V-in-olivine oxybarometry data for eleven MORB lavas, eight of which have been previously characterized by Fe-XANES oxybarometry. The new data show that the V-in-olivine method is not applicable to MORB with glass containing <8.3 wt. % MgO, and that MORB with glass having MgO >8.3 wt. % have an fO2 averaging -0.28 ± 0.28 ΔFMQ, overlapping with the global Fe-XANES MORB average of -0.18 ± 0.16 ΔFMQ. It is therefore recommended that this oxybarometry technique only be applied to MORB with primitive (>8.3 wt. % MgO) glass. The lack of any systematic offset between the data from the two oxybarometers supports our previous conclusion that ocean island basalt sources (OIB) are more oxidized than average MORB, and that the majority of Archean komatiite sources record significantly lower fO2 than modern MORB.

Revisiting the South China Sea MORBs: Mg isotope and whole-rock geochemical constraints

Mid-Ocean Ridge Basalts (MORBs) from the South China Sea (SCS) have attracted great attention from geologists in the past decade, but their petrogenesis still remains controversial. Here, we show that the Mg isotopes of MORBs from Integrated Ocean Drilling Program (IODP) Sites U1431, U1433 and U1434 are heterogeneous and present the mechanisms causing such heterogeneity. The MORBs from Sites U1431, U1433 and U1434 have δ26Mg values ranging from −0.35‰ to −0.21‰, from −0.34‰ to −0.13‰, and from −0.30‰ to −0.20‰, respectively. Together with all the published data, we found the MORBs at Site U1431 with high MgO contents (> 9 wt%) usually have δ26Mg values that are generally lower than the normal mantle (−0.25‰ ± 0.04‰), whereas some of the Site U1433 MORBs are higher than the normal mantle. Shallow-level geological processes like seawater alteration have negligible effects on the Mg isotope fractionation of our samples. Elemental ratio indicative of the degree of partial melting (like Sm/Nd) from the same site also shows no relationship with the δ26Mg variations. Olivine is proposed to have light Mg isotopes. Considering the high MgO (> 9 wt%) contents of low-δ26Mg samples and the petrographic evidence, we propose that their slightly light Mg isotopic compositions of U1431 MORBs may result from the enrichment of olivine, rather than reflecting the mantle source. Moreover, low Ce/Pb ratios and most variable δ26Mg values of some U1433 MORBs suggest that lower continental crustal materials have contributed to their source. As all the previously published MORBs are evolved, we use the Revpet to estimate the mantle melting conditions of SCS MORBs, including the Site U1500 samples recovered from the northern margin of SCS. Compared with the global MORBs data, there is no thermal anomaly in SCS MORBs. Together with our new Mg isotope data, therefore, we propose that the role of the Hainan mantle plume in the opening and spreading of the SCS may be insignificant.

Iron isotope heterogeneity in magmas of subduction zones and its origin

Iron isotope heterogeneity of subduction-related primitive magmas was traditionally thought to be predominantly attributed to the addition of slab-derived components. However, the specific subducted material responsible for inducing Fe isotope heterogeneity in the subarc-backarc mantle remains poorly constrained. Here we report Fe isotopic compositions of a series of backarc basin basalts (BABBs) from the Woodlark Basin (an intra-arc rift basin), Vate Trough (a nascent rear-rift basin), and Lau Basin (a relatively mature basin) in the southwestern Pacific. Even though these BABBs have various geochemical compositions ranging from mid-ocean ridge basalts (MORB)-like to arc-like, which should be affected by various amounts of the slab-derived fluids and/or sediment melts, their δ56Fe values (0.05–0.18‰) are within the range of global MORB (0.05–0.17‰). After correction for crystal fractionation, the primitive δ56Fe values (δ56Fe-prim) displayed no co-variation with indicators of subduction contributions (e.g., Ce/Pb, Nb/U, Ba/La, and Th/Yb) or source redox conditions caused by slab inputs (e.g., V/Yb, V/Ti), suggesting that the δ56Fe-prim values of these BABBs are rarely affected by subduction metasomatism and the change in oxygen fugacity caused by subducted input. Therefore, we proposed that the MORB-like δ56Fe values of the studied BABBs were primarily caused by mantle melting processes. Furthermore, we presented a compilation of available Fe isotopic data for BABBs and island arc basalts (IABs) and used these data to evaluate Fe isotope heterogeneity in subduction settings. In particular, compared with δ56Fe-prim of MORB, the δ56Fe-prim of BABBs from the Central Lau Spreading Centre in the Lau Basin and the IABs have lower values, whereas BABBs from the Rochambeau Ridges in the Lau Basin have higher δ56Fe-prim values. By comprehensively considering the factors affecting the variation in Fe isotopic compositions in magmatic rocks in subduction zones (e.g., crystal fractionation, partial melting of the mantle, redox conditions, and subduction-related metasomatism), we proposed that the varied Fe isotopic compositions in subduction settings could reflect their heterogeneous sources, which may be related to plate subduction. Specifically, during plate subduction, a melt-depleted refractory forearc mantle peridotite (with low δ56Fe values) could be dragged by subducted slab into subarc or backarc depths, resulting in light Fe isotopic enrichment in island arc magmas or BABBs; additionally, serpentinites (both in the slab and mantle wedge) can generate light Fe isotopic enrichment in island arc magmas. High δ56Fe values in the BABB (e.g., from the Rochambeau Ridges in the Lau Basin) may result from the toroidal influx of nearby upwelling plume components or the assimilation of hydrothermally altered oceanic crust.

Petrogenesis of high-alumina basalts: Implications for magmatic processes associated with the opening of the South China Sea

Results of International Ocean Discovery Program (IODP) Expeditions on the northern continental margin of the South China Sea (SCS) have nullified the early notion that the SCS is a magma-poor margin. However, there are continuing debates on how the rapid transition from continent to ocean is supported by geochemical and petrologic data, and whether such a process was related to plate subduction or mantle plume activities. Here we present the results of bulk-rock and plagioclase phenocryst geochemistry of the basalts generated during the early spreading stage (initial and steady ocean stages) of SCS basin extension, which were drilled at IODP Sites U1500 and U1503, respectively. We combined these findings with published data from the late spreading stage of the SCS basin, which provides an excellent opportunity to examine the magmatic processes associated with the evolution of the SCS basin. Our results revealed that the SCS basalts generally exhibit higher Al contents than global Mid-Ocean Ridge Basalts, particularly at lower MgO contents, which is similar to the characteristics of modern arc basalts. Nevertheless, their origins are site-specific and complex. For instance, some of the Site U1503 basalts originated through fractional crystallization of water-rich parental magmas with the lowest degree of melting, while the basalts from the other sites, including Site U1500, were primarily generated by the accumulation of plagioclase. More specifically, the water-rich magmas from Site U1503 displayed strong subduction signals in terms of trace elements, and their correlations with Al2O3 content suggest that they are the product of partial melting of the mantle wedge, closely related to the northward subduction of proto-SCS. This subduction had a significant effect on triggering the opening of the SCS, rather than a mantle plume. Moreover, the Site U1500 basalts exhibit a significant increase in plagioclase An values with the increase of host magma Al contents, indicating that the plagioclase floatation mechanism cannot account for the origin of their plagioclase accumulation. Therefore, we propose that the high abundance of plagioclase in Site U1500 basalts requires rapid ascent of magma, supporting a rapid rifting and strong magmatism during the initial opening of the SCS.

A Machine Learning Based‐Approach to Predict the Water Content of Mid‐Ocean Ridge Basalts

AbstractWater is critical in the evolution of the mantle due to its strong influence on the physicochemical properties of mantle rocks. Mid‐ocean ridge basalts (MORBs) are commonly used to study the compositional characteristics of the convecting upper mantle. However, there remains abundant samples in the global MORB data sets without direct measurements of water contents. The commonly observed good correlation between H2O and other incompatible trace components, such as Ce, has been applied to quantify water contents of MORBs. However, this approach assumes constant H2O/Ce in the target samples, which is not always true as the H2O/Ce ratios of MORBs could be rather heterogeneous even in some short ridge segments. Utilizing the present compositional data of global MORB glasses with measured water contents (n = 1,467), we construct a Random Forest Regression model based on machine learning, which can predict water concentrations of samples based on selected major and trace element data, without assuming a ratio between H2O and other trace elements. This model allows us to precisely recover water contents for MORBs with comparable accuracy with traditional analytical methods. The predicted results of MORB glasses from this model (n = 1,931) expand the water content database of global MORBs and indicate a broad existence of high‐H2O MORBs. This new approach allows us to investigate the water content of MORBs from some ridges lacking previous water content measurements (e.g., the Chile Ridge and the Pacific‐Antarctic Ridge) and infer changes in the water content of MORB sources through applying the model to transform fault samples.

Constraints of barium isotopes on recycling of ancient oceanic crust in the mantle of the South China Sea

As surface reservoirs (e.g. sediments and altered oceanic crust) have extremely high barium (Ba) concentrations and distinctive Ba isotope compositions compared to the mantle, Ba isotopes have the potential to trace crustal recycling in the mantle. The sediment components and fluids derived from altered oceanic crust in arc lavas and continental basalts have been identified using Ba isotopes, while the geochemical behavior of Ba in subducted oceanic crust remains enigmatic. A series of volcanic breccia that temporally evolved from carbonated silicate clasts to alkali basalts in the South China Sea (SCS) drilled by the Integrated Ocean Discovery Program (IODP) Site U1431 was suggested to originate from the mantle source contaminated by recycled oceanic crust. To understand the potential of using Ba stable isotopes to trace recycled oceanic crust in the mantle, we report high-precision Ba isotope compositions of these samples, together with alkali basalts from adjacent Hainan Island to constrain the Ba isotope characteristics of the Hainan mantle plume. The carbonated silicate clasts and alkali basalts from the SCS display extremely high Ba concentrations and are thus insensitive to magma evolution during ascent through the thin lithospheric mantle and proper to trace crustal materials in their mantle source using Ba isotopes. These samples have variable Ba isotope compositions ranging from δ138/134Ba = −0.10 to +0.05‰, implying local mantle isotopic heterogeneity underlying the SCS. The Ba isotope characteristics of some samples, which are much lighter than those of the mantle-derived carbonatites and global mid-ocean ridge basalts (MORBs), in conjunction with trace element ratios and radiogenic isotope compositions, reflect significant incorporation of recycled carbonated oceanic crust after subduction-related dehydration. The rising of the Hainan mantle plume offers the most reasonable interpretation of the entrainment of the recycled oceanic crust in the carbonated silicate magmatism beneath the SCS. Alkali basalts from Hainan Island display Ba isotope characteristics slightly heavier than the SCS carbonated silicate clasts and alkali basalts. It implies that the Hainan mantle plume underlying Hainan Island contains less recycled-oceanic-crust components than that underneath the SCS. This study highlights Ba isotopes as effective tracers of recycled oceanic crust in the mantle.

Re-visiting barium isotope compositions of mid-ocean ridge basalts and the implications

Barium (Ba) isotopes can be used as potential tracers for crustal material recycling in the mantle. Determination of the Ba isotope composition of the depleted mantle is essential for such applications. However, Ba isotope data for mantle-derived basalts are still rare. In this study, we reported high-precision Ba isotope data of 30 oceanic basalts including 25 mid-ocean ridge basalts (MORBs) from geochemically and geologically diverse mid-ocean ridge segments and five back-arc basin basalts. The δ&lt;sup&gt;138/134&lt;/sup&gt;Ba values of these samples varied from −0.06‰ to +0.11‰, with no systematic cross-region variation. Together with published data, we constrained the average δ&lt;sup&gt;138/134&lt;/sup&gt;Ba of global MORBs to +0.05‰±0.09‰ (2 standard deviation, &lt;i&gt;n&lt;/i&gt; = 51). Based on depleted MORBs that have (La/Sm)&lt;sub&gt;N&lt;/sub&gt; &lt; 0.8, low &lt;sup&gt;87&lt;/sup&gt;Sr/&lt;sup&gt;86&lt;/sup&gt;Sr (&lt; 0.70263), and low Ba/Th &lt; 71.3, we estimated the average δ&lt;sup&gt;138/134&lt;/sup&gt;Ba of the depleted MORB mantle (DMM) as + 0.05‰ ± 0.05‰ (2SD, &lt;i&gt;n&lt;/i&gt; = 16) that is significantly lower than the DMM (≈ 0.14‰) reported previously. If a new estimation of the DMM is applied, it is unreasonable to infer that the Ba isotope signatures of the “enriched-type” MORBs (E-MORBs) could be attributed to pervasive sediment recycling in the upper mantle. We, therefore, conclude that the Ba isotope compositions of the E-MORBs could be sourced from the incorporation of subducted altered oceanic crust and/or sediments depending on the Ba isotope composition and other geochemical information of the local mantle.

Global trends in novel stable isotopes in basalts: Theory and observations

The geochemistry of global mantle melts suggests that both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) sample lithological and temperature heterogeneities originating in both the upper and lower mantle. Recently, non-traditional stable isotopes have been suggested as a new tool to complement existing tracers of mantle heterogeneity (e.g., major and trace elements, radiogenic isotopes), because mineral- and redox-specific equilibrium stable isotope fractionation effects can link the stable isotope ratios of melts to their source mineralogy and melting degree. Here, we investigate five stable isotope systems (Mg-Ca-Fe-V-Cr) that have shown promise in models or natural samples as tracers of mantle temperature and/or lithological heterogeneity. We use a quantitative model, combining thermodynamically self-consistent mantle melting and equilibrium isotope fractionation models, to explore the behaviour of the isotope ratios of these elements during melting of three mantle lithologies (peridotite, and silica-excess and silica-deficient pyroxenites), responding to changes in mantle mineralogy, oxygen fugacity, temperature and pressure. We find that, given current analytical precision, the stable isotope systems examined here are not predicted to be sensitive to mantle potential temperature variations through equilibrium isotope fractionation processes. By contrast, source lithological heterogeneity is predicted to be detectable in some cases in the stable isotope ratios of erupted basalts, although generally only at proportions of >10% MORB-like pyroxenite in the mantle source, given current analytical precision. Magnesium and Ca stable isotopes show most sensitivity to a garnet-bearing source lithology, and Fe and Cr stable isotopes are potentially sensitive to the presence of MORB-like pyroxenite in the mantle source, although the behaviour of Cr isotopes is comparatively under-constrained and requires further work to be applied with confidence to mantle melts. When comparing the magnitude and direction of predicted equilibrium isotopic fractionation of peridotite and pyroxenite melts to natural MORB and OIB data, we find that aspects of the natural data (including the mean Mg-Ca-Fe-V isotopic composition of MORB, the range of Mg-Ca isotopic compositions seen in MORB data, the mean Mg-Ca-Cr isotopic composition of OIB, and the range of Mg-V-Cr isotopic compositions in OIB data) can be matched by equilibrium isotope fractionation during partial melting of peridotite and pyroxenite sources – with pyroxenite required even for some MORB data. However, even when considering analytical uncertainty on natural sample measurements, the range in stable isotope compositions seen across the global MORB and OIB datasets suggests that kinetic isotope fractionation, or processes modifying the isotopic composition of recycled crustal material such that it is distinct from MORB, may be required to explain all the natural data. We conclude that the five stable isotope systems considered here have potential to be powerful complementary tracers to other geochemical tracers of the source lithology of erupted basalts. However, continued improvements in analytical precision in conjunction with experimental and theoretical predictions of isotopic fractionation between mantle minerals and melts are required before these novel stable isotopes can be unambiguously used to understand source heterogeneity in erupted basalts.

K/U of the MORB Source and Silicate Earth

AbstractPotassium (K) informs on the radiogenic heat production, atmospheric composition, and volatile element depletion of the Earth and other planetary systems. Constraints on the abundance of K in the Earth, Moon, and other rocky bodies have historically hinged on K/U values measured in planetary materials, particularly comparisons of the continental crust and mid‐ocean ridge basalts (MORBs), for developing compositional models of the bulk silicate Earth (BSE). However, a consensus on the most representative K/U value for global MORB remains elusive despite numerous studies. Here, we statistically analyze a critical compilation of MORB data to determine the K/U value of the MORB source. Covariations in the log‐normal abundances of K and U establish that K is 3–7 times less incompatible than U during melting and/or crystallization processes, enabling inverse modeling to infer the K/U of the MORB source region. These comprehensive data have a mean K/U for global MORB = 13,900 ± 200 (2σm; n = 4,646), and define a MORB source region with a K/U between 14,000 and 15,500, depending on the modeled melting regime. However, this range represents strictly a lower limit due to the undefined role of fractional crystallization in these samples and challenges preserving the signatures of depleted components in the MORB mantle source. This MORB source model, when combined with recent metadata analyses of ocean island basalt (OIB) and continental crust, suggests that the BSE has a K/U value &gt;12,100 and contains &gt;260 × 10−6 kg/kg K, resulting in a global production of ∼3.5 TW of radiogenic heat today and 1.5 × 1017 kg of 40Ar over the lifetime of the planet.

Importance of the size and distribution of chemical heterogeneities in the mantle source to the variations of isotope ratios and trace element abundances in mid-ocean ridge basalts

The size and distribution of chemical heterogeneities in the mantle source affect compositions of basalts erupted on the surface. To understand the effect of size and distribution of heterogeneous mantle source on the variations of isotope ratios and incompatible trace element abundances in mid-ocean ridge basalts (MORB), we develop a time-dependent two-porosity ridge model that features disequilibrium melting and channelized melt migration in a truncated triangular melting region. We run numerical simulations for a two-component mantle that consists of the depleted mantle and enriched blobs spanning a range of enriched compositions and sizes. The volume fraction of enriched mantle varies under 10% of the mantle source and the size of the melt production area that contributes to the pooled melt is also treated as a variable. Results show that higher abundance of incompatible trace element in the enriched mantle, larger size of enriched blobs, and narrower melt pooling area produce larger variations of isotope ratios in the pooled melt. We develop an empirical scaling equation that quantifies the fluctuation of isotope ratios as a function of elemental abundance, size and average spacing of enriched heterogeneities in the source, and the width of melt pooling area. The two-standard deviation of 143Nd/144Nd, 176Hf/177Hf, 87Sr/86Sr, and 206Pb/204Pb in global MORB not affected by hotspots can be produced by melting a two-component mantle with 5.2–8.2% enriched heterogeneities of 5.1–6.7 km mean size and 7.0(±1.6), 5.3(±1.2), 2.0(±0.5), and 9.8(±2.2) times higher abundances of Nd, Hf, Sr, and Pb than the depleted mantle, respectively. Incomplete mixing of melts at the base of the lithosphere is likely important for producing the variation of incompatible trace elements in MORB. The melt erupted at the ridge axis may represent an ensemble of the melt pooled from the center of the melt pooling area and melts pooled from local areas away from the ridge axis. The present study underscores the importance of the size and distribution of mantle heterogeneities to the interpretation of isotope ratios and trace element abundances in basalts.

Heterogeneous mantle melting and magmatic processes at the East Pacific Rise (2.6–3.1°S): Evidence from mid-ocean ridge basalt geochemistry and Sr–Nd–Pb isotopes

ABSTRACT We present the major and trace elements and Sr, Nd, and Pb isotopes in mid-ocean ridge basalts (MORB) from the East Pacific Rise (EPR) at 2.6–3.1°S. These samples are low-K tholeiites and show significant variation in their major element compositions (e.g. 4.60–8.18 wt% MgO, 8.34–12.12 wt% CaO, 9.78–14.25 wt% Fe2O3, and 0.06–0.34 K2O wt%). Trace element abundances of the 2.6–3.1°S MORB are variably depleted (e.g. (La/Sm), N = 0.51–0.78, Zr/Y = 2.35–3.42, Th/La = 0.035–0.056, and Ce/Yb = 2.38–3.96) but closely resemble the average N-MORB. In the compatible elements (Ni and Cr) against incompatible element Zr plots, the 2.6–3.1°S MORB show well-defined negative correlations, together with a liquid line of descent (LLD) modelling and petrographic observations, implying a significant role of olivine, plagioclase and clinopyroxene fractionation during magma evolution. When compared to global MORB and peridotites, the 2.6–3.1°S MORB and most of the other axial lavas from the South EPR show similar Zn/Fe, Zn/Mn, and Fe/Mn ratios, attesting to a peridotite-dominated mantle lithology. However, the relationships between incompatible trace element ratios, such as Zr/Rb and Nb/Sm, and the negative correlation between Zr/Nb and 87Sr/86Sr indicate a geochemically heterogeneous mantle source. The mantle beneath the South EPR likely consists of two components, with the enriched component residing as physically distinct domains (e.g. veins or dikes) in the depleted peridotite matrix. In the Sr–Nd–Pb isotope space, the South EPR MORB lie along the mixing lines between the depleted MORB mantle (DMM) and the ‘C’-like Pukapuka endmember. We infer that low-F melts derived from these enriched materials may cause localized mantle heterogeneity (veins or dikes) via an infiltration process. Subsequent melting of the refertilized mantle may impart an isotopically distinct characteristic to South EPR MORB.

Carbon Fluxes and Primary Magma CO2Contents Along the Global Mid‐Ocean Ridge System

AbstractThe concentration of carbon in primary mid‐ocean ridge basalts (MORBs), and the associated fluxes of CO2outgassed at ocean ridges, is examined through new data obtained by secondary ion mass spectrometry (SIMS) on 753 globally distributed MORB glasses. MORB glasses are typically 80–90% degassed of CO2. We thus use the limited range in CO2/Ba (81.3 ± 23) and CO2/Rb (991 ± 129), derived from undegassed MORB and MORB melt inclusions, to estimate primary CO2concentrations for ridges that have Ba and/or Rb data. When combined with quality‐controlled volatile‐element data from the literature (n = 2,446), these data constrain a range of primary CO2abundances that vary from 104 ppm to 1.90 wt%. Segment‐scale data reveal a range in MORB magma flux varying by a factor of 440 (from 6.8 × 105to 3.0 × 108m3/year) and an integrated global MORB magma flux of 16.5 ± 1.6 km3/year. When combined with CO2/Ba and CO2/Rb‐derived primary magma CO2abundances, the calculated segment‐scale CO2fluxes vary by more than 3 orders of magnitude (3.3 × 107to 4.0 × 1010mol/year) and sum to an integrated global MORB CO2flux of × 1012mol/year. Variations in ridge CO2fluxes have a muted effect on global climate; however, because the vast majority of CO2degassed at ridges is dissolved into seawater and enters the marine bicarbonate cycle. MORB degassing would thus only contribute to long‐term variations in climate via degassing directly into the atmosphere in shallow‐water areas or where the ridge system is exposed above sea level.

High-precision measurement of B isotopes on low-boron oceanic volcanic rock samples via MC-ICPMS: Evaluating acid leaching effects on boron isotope compositions, and B isotopic variability in depleted oceanic basalts

Recent analytical advances have made the high precision (2SD, ±≤1‰) determination of boron (B) isotopic ratios in silicate rocks a possibility, but few such data are as yet available, especially for low B (<2 ppm) samples like oceanic basalts, where issues related to cryptic seawater alteration are a concern. This study carefully assesses the reproducibility of B isotope measurement via MC-ICPMS on silicate rocks. Based on repeated measurements of the standard reference materials B-5, B-6, JB-2, JR-2, AGV-2 and JB-3 and nature samples, the external reproducibility of MC-ICPMS measurements on the same solution is generally between 0.1‰ and 0.5‰ (2SD), a similar level of accuracy to that obtained based on long-term measurement of solution standard NIST SRM 951 of 0.4‰ (2SD). For different digestions, the average external reproducibility is 0.6‰ (2SD), which represents the whole procedure reproducibility, including chemical treatment. We have tested an acid leaching experimental procedure for its effects on the B isotope compositions of ~50 Ma boninites collected from a deep ocean setting. Fresh and moderately altered boninite samples were subjected to cold HCl leaching treatments for 1, 3, 5, 7 and 9 h, using 2 N and 6 N HCl, respectively. δ11B values for the fresh sample remained unchanged within error after leaching, suggesting that leaching did not lead to any B isotopic fractionation. δ11B values for different aliquots of an altered boninite sample show lower and uniform δ11B values after 1 h of 6 N HCl leaching, while aliquots leached in 2 N HCl show similar δ11B to the unleached altered sample. These results suggest that changes in B isotopes caused by at least moderate degrees of alteration, and/or adsorption effects in volcanic rocks preserved on the ocean floor can be effectively eliminated through short-duration 6 N HCl leaches before digestion and analysis. B isotope measurements on three documentably fresh, low-B N-type mid-ocean ridge basalt (MORB) glass samples from the Southwest Indian Ridge yield δ11B values from −6.62‰ to −7.39‰, within the range of the recommended global MORB value of ≈−7.1 ± 0.9‰, determined via SIMS methodologies (2SD at ±1.5‰; Marschall and Monteleone, 2015), and the range of four fresh glass samples from the Eastern Pacific Ridge of −7.3 ± 0.8‰, measured by in-situ laser-ablation ICP-MS (2SD at ±0.7–1.0‰; Le Roux et al., 2004).

Mantle Source and Magmatic Evolution of the Dying Spreading Ridge in the South China Sea

AbstractThe genesis of seamount chains along fossil spreading ridges reflects the mantle melting dynamics of dying spreading center. The South China Sea developed a seamount chain along its fossil ridge shortly after cessation of spreading. International Ocean Discovery Program Expedition 349 has recovered three volcanic layers, a shallow volcanic breccia layer and two deeper (upper and lower sections) basalt layers, at Site U1431 near the fossil ridge of the South China Sea. Despite the mid‐ocean ridge basalt (MORB)‐like geochemistry of the lower and upper section basalts, they show distinct geochemical differences that indicate a change of mantle source and a transition from spreading to intraplate volcanism. While the lower section basalts could reflect the last stage volcanism before the cessation of ridge spreading, the upper section basalts with whole‐rock MgO up to ~20 wt.% were formed by intraplate volcanism with significant olivine accumulation once the spreading center had become inactive. The MORB type basalts at Site U1431 have Hawaii‐like high olivine Ni and Fe/Mn and low olivine Ca and Mn and bulk‐rock low CaO that are distinctly different from normal global MORBs, suggesting an anomalous mantle source in lithology. We suggest that an eclogite‐/pyroxenite‐rich component, possibly sourced from the Hainan hot spot, played a fundamental role in the transition from spreading to intraplate volcanism of the dying spreading ridge of the South China Sea.

The behaviour of ferric iron during partial melting of peridotite

In order to better understand the behaviour of Fe3+ during partial melting of a mid-ocean-ridge-basalt (MORB) mantle source, we performed partial melting experiments of a spinel peridotite at 1.5 GPa, between 1320 and 1450 °C, and over a range of oxygen fugacities (fO2) varying from FMQ-3.5 to FMQ+6 (log-units relative to FMQ = Fayalite-Magnetite-Quartz oxygen buffer). Changes in fO2 were achieved using different capsule materials and by varying the oxidation state of the starting material. Fe3+ contents of spinels and glasses were examined using Fe K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy in full-field mode. Our results show that this method allows Fe3+/ƩFe measurements to be made of spinels in our experimental charges but is not suitable for quantifying Fe3+/ƩFe of glasses, particularly at relatively low fO2 where melt Fe2O3 contents are small. We therefore employed several methods (Fe3+/ƩFe ratios of spinels measured by XANES and calculated by stoichiometry, and Fe-alloy sliding redox sensors) to assess the fO2 of the samples. These were then used to calculate the Fe3+/ƩFe ratios of glasses using published models, which were further constrained by Mössbauer spectroscopy measurements on an additional set of partial melting experiments.For a particular initial redox state, the Fe3+/ΣFe ratio in melts remains approximately constant with partial melting degree. This is explained by a slight decrease of the bulk partition coefficient of Fe2O3 (DFe2O3peridotite-melt) as the degree of partial melting increases. The Fe3+/ΣFe ratios of resulting melts are thus restricted to a relatively narrow range over large degrees of melting, as observed for global MORB glass analyses. This observation is also confirmed by pMELTS calculations employing different bulk Fe3+/ΣFe ratios. We conclude that the Fe2O3 content in MORB can be accounted for through partial melting of a mantle source containing 0.1–0.5 wt% Fe2O3 and a bulk DFe2O3peridotite-melt ranging from 0.3 to 0.1.

Reconstructing mantle carbon and noble gas contents from degassed mid-ocean ridge basalts

The fluxes of volatile elements from the mantle have long been used to understand mantle structure and evolution, and are critical controls on Earth's climate stability. Because of the ubiquity of magmatic degassing, inferring pre-degassing volatile concentrations from measured basalts requires the application of a degassing model. Such models, including the commonly-applied equilibrium Rayleigh distillation, typically assume equilibrium or solubility-based partitioning between melt and vapor. Here, we demonstrate that ratios of radiogenic isotopes of He, Ne, Ar and, especially, Xe measured in global mid-ocean ridge basalts (MORBs) are inconsistent with equilibrium degassing models, even when not considering He. We conclude that kinetic disequilibrium is a crucial process affecting volatile abundances during degassing. We present a simple disequilibrium Rayleigh distillation model to reconstruct pre-degassing MORB noble gas and carbon concentrations, which predicts that He and Ne achieve nearly equilibrium partitioning between melt and vesicles, but the slower-diffusing heavier noble gases are strongly affected by disequilibrium, resulting in non-equilibrium fractionation of noble gas elemental ratios, and other volatile element ratios like CO2/He.We apply our model to a large set of MORB data, and find average pre-degassing 3He, 22Ne, and 36Ar concentrations of 4.4 ± 0.9 × 10−10, 6.6 ± 1.4 × 10−11, and 6.8 ± 4.5 × 10−10 ccSTP/g (2σ), with variations of approximately 2 orders of magnitude, similar to other highly incompatible elements. Pre-degassing noble gas concentrations imply a mid-ocean ridge 3He flux of 800 ± 170 mol/yr and upper mantle 3He/22Ne and 3He/36Ar ratios of 6.6 ± 2.0 and 0.64 ± 0.44, but substantial variability in these ratios between samples. Applying our model to CO2, we calculate an average mantle CO2/3He molar ratio of 1.67 ± 0.21 × 109, which, when combined with our estimate of 3He flux, implies an upper mantle CO2 flux of 5.9 ± 1.0 × 1013 g/yr and a CO2 concentration of approximately 110 ppm.Our estimate of the mantle 3He flux is the first determined independently of oceanographic 3He measurements, and consequently represents a time-integrated flux substantially longer than ∼1000 years. And although at the high end of the range of previous estimates, our estimate does not resolve the long-standing heat-helium paradox. Additionally, our requirement for heterogeneous pre-degassing 3He/22Ne and 3He/36Ar ratios between samples is contrary to conclusions of previous applications of disequilibrium degassing models, which advocated uniform ratios. Furthermore, we find that CO2/Ba ratios are highly variable in MORB samples, but still consistent with an average mantle mass ratio of ∼100. However, estimating pre-degassing CO2 concentrations and the mantle CO2 flux depend strongly on the poorly-constrained carbon diffusivity. Consequently, our demonstration of the prevalence of disequilibrium during mid-ocean ridge degassing, and the potential for disequilibrium in other volcanic settings, highlights the need for better characterization of the physical parameters associated with volcanic degassing.

A Statistical Description of Concurrent Mixing and Crystallization during MORB Differentiation: Implications for Trace Element Enrichment

The pattern of trace element enrichment and variability found in differentiated suites of basalts is a simple observable, which nonetheless records a wealth of information on processes occurring from the mantle to crustal magma chambers. The incompatible element contents of some mid-ocean ridge basalt (MORB) sample suites show progressive enrichment beyond the predictions of simple models of fractional crystallization of a single primary melt. Explanations for this over-enrichment have focused on the differentiation processes in crustal magma chambers. Here we consider an additional mechanism and focus instead on the deviation from simple fractionation trends that is possible by mixing of diverse mantle-derived melts supplied to magma chambers. A primary observation motivating this strategy is that there is significant chemical diversity in primitive high-MgO basalts, which single liquid parent models cannot match. Models were developed to simulate the compositional effects of concurrent mixing and crystallization (CMC): diverse parental melts were allowed to mix, with a likelihood that is proportional to the extent of fractional crystallization. Using a simple statistical model to explore the effects of concurrent mixing and crystallization on apparent liquid lines of descent, we show how significant departure from Rayleigh fractionation is possible as a function of the diversity of trace elements in the incoming melts, their primary MgO content, and the relative proportion of enriched to depleted melts. The model was used to make predictions of gradients of trace element enrichment in log[trace element]–MgO space. These predictions were compared with observations from a compilation of global MORB and provide a test of the applicability of CMC to natural systems. We find that by considering the trace element variability of primitive MORB, their MgO contents and degree of enrichment, CMC accurately predicts the pattern of trace element over-enrichment seen in global MORB. Importantly, this model shows that the relationship between over-enrichment and incompatibility can result from mantle processes: the fact that during mantle melting maximum variability is generated in those elements with the smallest bulk Kd. Magma chamber processes are therefore filtering the signal of mantle-derived chemical diversity to produce trace element over-enrichment during differentiation. Finally, we interrogate the global MORB dataset for evidence that trace element over-enrichment varies as a function of melt supply. There is no correlation between over-enrichment and melt supply in the global dataset. Trace element over-enrichment occurs at slow-spreading ridges where extensive steady-state axial magma chambers, the most likely environment for repeated episodes of replenishment, tapping and crystallization, are very rarely detected. This supports a model whereby trace element over-enrichment is an inevitable consequence of chemically heterogeneous melts delivered from the mantle, a process that may operate across all rates of melt supply.

The Meaning of Global Ocean Ridge Basalt Major Element Compositions

Mid-ocean ridge basalts (MORB) are arguably the most abundant and also the simplest igneous rocks on the Earth. A correct understanding of their petrogenesis thus sets the cornerstone of igneous petrogenesis in general and also forms the foundation for studying mantle dynamics. Because major element compositions determine the mineralogy, phase equilibria and physical properties of rocks and magmas, understanding global MORB major element systematics is of prime importance. The correlated large MORB major element compositional variations are well understood as the result of cooling-dominated crustal-level processes (e.g. fractional crystallization, magma mixing, melt–rock assimilation or reaction, and other aspects of complex open-magma chamber processes), but it remains under debate what messages MORB major elements may carry about mantle sources and processes. To reveal mantle messages, it is logical to correct MORB melts for the effects of crustal-level processes to Mg# ≥ 0·72 to be in equilibrium with mantle olivine of ≥Fo90. Such corrected MORB major element (e.g. Si72, Ti72, Al72, Fe72, Mg72, Ca72 and Na72) compositional variations thus reflect fertile mantle compositional variation, composition-controlled mantle physical property variation (e.g. density and solidus), variation in the extent and pressure of melting, and uncertainties associated with the correction. The correction-related uncertainties can be removed through justified heavy averaging. Because ridge axial depth variation (∼0 to ∼6000 m below sea level) and plate spreading rate variation ( 150 mm a–1) are the two largest known physical variables along the global ocean ridge system, possible correlations of MORB major element compositions at Mg# ≥ 0·72 with these two physical variables are expected to reveal intrinsic controls on global MORB petrogenesis and ocean ridge dynamics. Indeed, global MORB major element data averaged with respect to both ridge axial depth intervals and ridge spreading rate intervals show significant first-order correlations. These correlations lead to the conclusion that the ridge axial depth variation and MORB chemistry variation are two different effects of a common cause, induced by fertile mantle compositional variation. The latter determines (1) variation in both composition and mode of mantle mineralogy, (2) variation of mantle density, (3) variation of ridge axial depth, (4) source-inherited MORB compositional variation, (5) density-controlled variation in the maximum extent of mantle upwelling, (6) apparent variation in the extent of melting, and (7) the correlated variation of MORB chemistry with ridge axial depth. These correlations also confirm the recognition that the extent of mantle melting increases with, and is caused by, increasing plate spreading rate. Mantle temperature variation could play a part, but its overstated role in the literature results from a basic error (1) in treating ridge axial depth variation as solely caused by mantle temperature variation by ignoring the intrinsic control of mantle composition, (2) in treating mantle plume-influenced ridges (e.g. Iceland) as normal ridges of plate spreading origin, and (3) in treating seismic low velocity at great depths (>300 km) beneath these mantle plume-influenced ridges as evidence for hot ridge mantle. There is no evidence for large mantle temperature variation beneath ridges away from mantle plumes. The suggested conclusions of this study may continue to be debated, but they are most objective, and are most consistent with petrological, geochemical, geological and geophysical principles and observations.

Melt generation beneath Arctic Ridges: Implications from U decay series disequilibria in the Mohns, Knipovich, and Gakkel Ridges

We present new 238U–230Th–226Ra–210Pb, 235U–231Pa, and Nd, Sr, Hf, and Pb isotope data for the slow- to ultraslow-spreading Mohns, Knipovich, and Gakkel Ridges. Combined with previous work, our data from the Arctic Ridges cover the full range of axial depths from the deep northernmost Gakkel Ridge shallowing upwards to the Knipovich, Mohns, and Kolbeinsey Ridges north of Iceland. Age-constrained samples from the Mohns and Knipovich Ridges have (230Th/238U) activity ratios ranging from 1.165 to 1.30 and 1.101 to 1.225, respectively. The high 230Th excesses of Kolbeinsey, Mohns, and Knipovich mid-ocean ridge basalts (MORB) are erupted from ridges producing relatively thin (Mohns, Knipovich) to thick (Kolbeinsey) oceanic crust with evidence for sources ranging from mostly peridotite (Kolbeinsey) to eclogite-rich mantle (Mohns, Knipovich). Age-constrained lavas from 85°E on the Gakkel Ridge, on the other hand, overlie little to no crust and range from small (∼5%) 230Th excesses to small 238U excesses (∼5%). The strong negative correlation between (230Th/238U) values vs. axial ridge depth among Arctic ridge basalts is controlled not only by solidus depth influence on 238U–230Th disequilibria, but also by variations in mantle source lithology and depth to the base of the lithosphere, which is expected to vary at ultra-slow spreading ridges. Small 231Pa excesses (65% excess) in age-constrained basalts support the presence of eclogite in the mantle source for this region. Conversely, the ultraslow-spreading Gakkel Ridge basalts are homogeneous, with Sr, Nd, and Hf radiogenic isotopic signatures indicative of a long time-averaged depleted mantle source. The Gakkel samples have minimum (226Ra/230Th) ratios ranging from 3.07 to 3.65±3%, which lie along and extend the global negative correlation between 226Ra and 230Th excesses observed in MORB. The new 230Th–226Ra data support a model for global MORB production in which deep melts record interaction with shallower materials. This scenario requires either mixing with shallow-derived melts, or melt-rock reaction with shallower rocks in the lithosphere or crust.